1. Field of the Invention
This invention relates generally to oxygen generating compositions, and more particularly concerns improved oxygen generation compositions containing copper, zinc, and antimony as a fuel.
2. Description of Related Art
Chemical oxygen generators are typically used in situations requiring the generation of supplemental oxygen, such as in aviation during accidental decompression, in submarines, and in other similar settings. Oxygen for such purposes must be of suitably high purity. For example, the requirements of SAE Aerospace Standard AS801OC are frequently applied to oxygen utilized in aviation applications. Chemical oxygen generating compositions based upon the decomposition of alkali metal chlorates or perchlorates have long been used as an emergency source of breathable oxygen, such as in passenger aircraft, for example. Oxygen generating compositions utilizing alkali metal chlorates or perchlorates are disclosed in U.S. Pat. Nos. 5,198,147; 5,279,761; and 5,298,187; each of which are incorporated herein by reference.
A typical chemical oxygen generating candle may have several layers with different compositions to obtain different reaction rates and flow rates which are desired at different stages of operation. Multiple layers with different formulas are used in the candle to meet the oxygen generation requirements, which can vary with time, such as when an airplane descends following a loss of cabin pressure. An aircraft oxygen generator is commonly preprogrammed according to the descent profile of a given type of airplane, and must meet the minimum oxygen flow requirement at all times during a descent.
The candle typically has a generally cylindrical shape with a taper, with a recess at one end to hold an ignition pellet. The ignition pellet is ignited by firing a primer, and heat from the ignition pellet then initiates the reaction of the candle body, generating oxygen.
In a typical chemical oxygen generator, a sodium chlorate candle is encased in a stainless steel canister, and oxygen is generated by decomposition of sodium chlorate in the presence of a commonly used fuel, such as iron, and a catalyst, which is commonly a transition metal oxide such as the oxide of cobalt, which is an expensive, strategic metal. The principal heat producing reactions are described by the following equations: EQU NaClO.sub.3 (s).fwdarw.NaCl(s)+(3/2)O.sub.2 (g)+Heat(14,731 cal) EQU 2Fe(s)+(3/2)O.sub.2 (g).fwdarw.Fe.sub.2 O.sub.3 (s)+Heat(198,500 cal)
While iron powder has commonly been used in chlorate or perchlorate compositions as a fuel or catalyst for decomposition of the chloratcs or perchlorates to release oxygen, such iron powder typically contains small amounts of carbon (0.02% to 1%) that can at times also contaminate the oxygen released with up to 1,000 ppm of carbon monoxide, as described in the following equations, and which must be removed to provide a safely breathable gas. EQU C+O.sub.2 .fwdarw.CO.sub.2 EQU C+1/2O.sub.2 .fwdarw.CO
The production of carbon monoxide by alkali metal chlorate or perchlorate compositions utilizing iron powder as a fuel can be explained by the fact that iron has a high affinity for carbon, and can form various compounds with carbon. Iron powder usually contains several hundred to several thousand ppm carbon. Upon oxidation of the iron powder in a chemical oxygen generator, part of the carbon content of the iron powder is converted to carbon monoxide at unacceptably unhealthy levels, so that the carbon monoxide must be removed from the oxygen produced. Above 710.degree. C., thermodynamic constraints favor CO formation over formation of CO.sub.2. Since iron is a very energetic fuel (1,777 cal/g), and loading can be relatively high in some portions of the candle, temperatures in excess of 710.degree. C. can easily be reached. Even after oxygen evolution has ceased in those sections of the candle, temperatures typically continue to rise due to the oxidizing environment that is produced and can increase the extent of oxidation of iron. Thus, high levels of carbon monoxide in the oxygen produced by the initial stages of a candle fueled by carbon-containing metal powders such as iron, in excess of a maximum permissible level of 50 ppm, are common.
Carbon-free iron powder is difficult to obtain and expensive, so that in order to use iron powder as a fuel in an oxygen generator, it is necessary and economically preferable to utilize a Hopcalite filter to convert the carbon monoxide produced to the less toxic carbon dioxide. Unfortunately, the equipment necessary to filter and convert carbon monoxide to carbon dioxide not only increases manufacturing costs, but can also add up to 50 grams to the weight of an iron fueled oxygen generation system, which can be a considerable disadvantage if the oxygen generation system is to be used on board aircraft. The non-iron fuels manganese and silicon are not desirable, because they typically produce large amounts of highly toxic chlorine. As another alternative, oxygen generating compositions with no fuel are known, instead having high loadings of extremely moisture sensitive catalysts such as sodium oxide (Na.sub.2 O) to compensate for the lack of fuel. The generator formulation is dry and fuel free, to avoid the necessity of providing a filter, but such non-fueled oxygen generators are generally less reliable than fueled generators; and a non-fueled oxygen generator has difficulty achieving a full rate of flow within ten seconds during a cold test.
There is thus a need for a reliable, robust oxygen generator composition utilizing substantially carbon-free alternative fuels which are not as energetic as iron, do not produce high levels of chlorine characteristic of fuels such as manganese and silicon, to reduce or eliminate formation of toxic carbon monoxide that is formed due to carbon in metal powder used as a fuel for the oxygen generation process, to eliminate the need for a carbon monoxide Hopcalite filter and the possibility of carbon dioxide formation associated with such a traditional filter, and to generate oxygen at lower temperatures than in traditional iron fueled oxygen generators. The present invention meets these needs.